It is customary in food packaging to vacuum package bulky food items in heat shrinkable bags. After evacuation and sealing, the bag sometimes referred to as a "shrink bag", is immersed in hot water or otherwise exposed to heat which causes the bag to shrink tightly about the food item to create a relatively wrinkle-free package. Fresh and frozen turkeys and the like are commonly packaged in this fashion for retail sale.
Since the resulting package is relatively bulky and heavy, a handle to facilitate lifting and carrying the package is desirable. A shrink bag having a handle portion at the preclosed end of the bag is disclosed in U.S. Pat. No. 5,120,553. This bag has a skirt of heat shrinkable material at the preclosed end of the bag. The skirt is composed of a part of each of two superimposed bag panels so it is two plies thick. There is a slit cut in the skirt. On heat shrinking about a product sealed in the bag, the skirt shrinks and thickens to provide a handle for carrying the resulting package wherein the slit forms the handle opening. The easiest and most conventional way of making the slit is to use a hot wire to burn through both plies of the skirt material. The hot wire melts the plies so they weld together at the edge of the slit. The result is that the slit edge is a fused bead formed from both skirt plies.
The fused bead produced by the hot wire burning through the plies also acts as a seal to prevent entry of liquids or contaminants into the area between the plies. However, one drawback of using a hot wire to form the slit is that the resulting fused bead at the slit edge detracts from the handle appearance on subsequent heat shrinking. In this respect, when the bag material undergoes heat shrinking the fused bead at the slit edge shrinks much less than the surrounding plies. As a result, the shrinkage of the surrounding plies draws the fused bead together lengthwise so it assumes a crinkled appearance. Since the fused bead becomes an edge of the bag handle, the effect is that the handle is crinkled along its edges which detracts from the appearance of the handle.
The crinkled appearance of the edge of the bag handle can be avoided by cutting the slit with a cold knife. With a cold cut slit, the plies at the slit edge are not fused together so no bead is formed. Without a fused bead, the heat shrinking is uniform along the length of the slit so the resulting handle has smoother, more attractive side edges.
However, there are drawbacks to cutting the slit with a cold knife. For example, with a cold knife the slit edge may have nicks along the slit edge. These nicks represent stress concentrations which can compromise the strength of the handle. This is because if a nick is put into tension, as may occur when the handle is used for lifting, a tear will initiate at the nick and propagate through the handle causing it to fail. Nicks in the skirt material around the ends of the slit are a particular problem since these areas are most likely to be subjected to tensile forces. It is conventional to curve the ends of the slit to increase handle strength. However, with all things being equal, it has been found that handles formed using a cold knife to cut the slit, are much weaker than if a hot wire is used to form the slit. Also, with a cold cut slit the plies of film forming the handle are not welded together. This allows free passage and entry of liquids and contaminants into the space between the plies.